This invention relates to a rotating anode for X-ray tubes. More particularly, the invention relates to a rotating anode for X-ray tubes which has a basic member made of carbonaceous molybdenum alloy, such as TZM, and having a focal path, that is a cathode path, of tungsten or a tungsten alloy, the surface of the basic member outside the focal path being coated at least partially with one or more oxides or a mixture of one or more metals and one or more oxides.
The electric energy conveyed to a rotating anode in the production of X-rays is converted into X-ray radiation energy in the amount of appoximately only 1%. The remaining 99% is converted into undesirable heat, which leads to a heavy temperature load. For this reason, many attempts in the past have been made to carry off the thermal energy that is generated in rotating anodes as quickly as possible. In the main such attempts have involved increasing the surface thermal emissivity. Known ways of accomplishing this are coating of the rotating anode with graphite, with layers of pulverized refractory metals such as, for example, titanium or tantalum, or of carbides such as, for example, titanium carbide or tantalum carbide, or of oxide mixtures or oxide-metal mixtures.
West German Offenlegungsschrift No. 2443354 discloses a rotating anode of the kind mentioned above in which the basic member which may be made of TZM, for example, in order to increase the thermal radiation capability, is coated with a metal oxide layer of aluminum oxide and titanium oxide.
Austrian Pat. No. 336,143 likewise discloses a rotating anode having a basic member made of refractory metals, as well as, for example, molybdenum alloys and which anode is provided outside the focal path with a covering layer or coating of a composite of molybdenum and/or tungsten and/or niobium and/or tantalum with oxide ceramic materials, such as TiO.sub.2 and/or Al.sub.2 O.sub.3 and/or ZrO.sub.2.
Therefore, in both of the above mentioned publications, carbonaceous molybdenum alloys are suggested or expressly mentioned as the basic material to be employed in the basic member. Hence, on the basis of these publications, it was obviously neither expected nor perceived by those skilled in the art that by employing a covering layer which was suitable in other cases, the expected thermal radiation increase lasting as long as the usual lifetime could not be achieved in the case of carbonaceous molybdenum alloys, especially TZM.
On the contrary, however, Applicants have found, altogether surprisingly, that in the case of rotating anodes having a basic member made of a carbonaceous molybdenum alloy, especially TZM, and which is furnished with a coating of oxides to increase the thermal radiation, exhibits severe deterioration of the originally good emission characteristics after the anode is in operation a short time. While this phenomenon is probably attributed to carbon diffusion from the basic member into the outer oxide layer, the negative influence on the thermal radiation capability still is not understandable, since it is just as well known and a usual procedure, according to the state of the art, to apply pure carbide layers, such as titanium carbide, to rotating anode basic members to increase thermal radiation.
There exists, therefore, a need for rotating anodes for X-ray tubes such as those mentioned above but which do not exhibit the disadvantageous properties thereof. It is, therefore, an object of this invention to produce a rotating anode for X-ray tubes having a basic member made of carbonaceous molybdenum alloys and in which an increased thermal emissivity is achieved independently of the length of time the anode is in operation.